Process of hardening metal surfaces



Dec. 26, 1967 H- BLASZKOWSK! 3 5 PROCESS OF HARDENING METAL SURFACES Original Filed March 4, 1963 [2 V, POWER SUPPLY R\\LJ -\4/2JL P7 4 INVENTOR.

HENRYBL/ISZKOWSK/ United States Patent 262,655, 19, 1966, Ser. No.

8 Claims. (Cl. 219-76) This invention relates to methods for hardening metal surfaces by subjecting them to electrical discharges and then burnishing them with a hard, dull tool. This application is a continuation of application Ser. No. 262,655 filed Mar. 4, 1963, now abandoned, which in turn was a continuation-in-part of application Ser. No. 822,767 filed June 25, 1959, now abandoned.

In my patent applications Ser. No. 184,473 filed Apr. 2, 1962; Ser. No. 195,918 filed May 18, 1962; Ser. No. 218,486 filed Aug. 20, 1962; Ser. No. 238,314 filed Oct. 23, 1962; the aforesaid Ser. No. 822,767 and Ser. No. 853,883 filed Nov. 18, 1959 (all now abandoned but the subject matter of which is in pending applications Ser. No. 412,407 filed Nov. 19, 1964 and Ser. No. 488,626 filed Sep. 20, 1965) I have described methods of hard coating metal surfaces by subjecting them to localized electrical discharges from an electrode, either in the presence of a special surface preparation such as a carbide powder or through using an electrode formed of a material such as carbide which is capable of interacting with the surface metal. Broadly, such methods comprise establishing a potential difference between the surface and the electrode, the electrode being brought into contact with the surface and some relative motion being introduced between the two so that successive localized electrical discharges occur about the surface with resultant intense localized heating.

Surfaces prepared in this manner can exhibit two characteristics which limit their range of application. First, the heat associated with the discharges can tend to produce a softened layer in the base metal immediately below the hardened top layer. If steel is the metal being treated this softened zone generally exhibits a high austenite content. When used on tools, the hardened upper layer can wear through, exposing the soft base which exhibits very poor wear qualities.

The second undesirable property of metals treated by the electrode hardening process is the roughness of the upper coating. The process is inherently an intermittent one as the hardness producing discharges are repeatedly initiated and extinguished. Therefore, the surface coating is not regular in character, and although it is normally continuous, its upper surface often exhibits an undesirable roughness.

In order to eliminate the shortcomings of the discharge processing, I have added a second stage to the process which eliminates the disadvantages of a soft sub-surface zone and high surface roughness and actually produces a superior coating to one that would have resulted had these undesirable properties not occurred in the first stage. In essence the second stage which I add to the process consists of rubbing or burnishing the discharged deposited coating with a smooth, dull, hard tool under high pressure. The existence of the softened layer beneath the surface allows a plastic flow under the pressure of the finishing tool so that the peaks of the top layer may be displaced. The same flow assists the hardening of the austenite in the treated zone. In order to maintain a high pressure, the finishing tool only contacts the coated surface at a point or along a line. A tool geometry such as a roller is used to achieve this result. This finishing operation tends to hard work the austenitic sub-surface layer so as to transform it to a martensitic form. It also tends 3,360,630 Patented Dec. 26, 1967 to flatten and lay over the microscopic peaks of hardened metal of the upper surface. Therefore, the hardened upper layer is not in the least diminished as it would be if thle surface were treated by conventional polishing metho s.

While tools which have been coated in accordance with the first stage of my process but not smoothed in accordance with my second stage might be used in such a manner that the cutting operation in which they were employed performed the hard working, I have found that such change is only achieved under particular sets of environmental conditions such as materials, feeds, speeds, lubricants, etc. Therefore, the possibility of obtaining the desirable hard working in a particular tool is a mere matter of chance.

It is therefore seen to be an object of the present invention to provide a method of hardening metal surfaces which comprises two stages, a first wherein an electrode is brought into contact with the surface and a potential difference is established between the electrode and the work so that electrical discharges occur as the electrode is moved relative to the work; and a second step wherein a smooth hard tool is brought into pressured contact with the surface and relative motion is provided between the tool and the surface.

Other objects, advantages and applications of the present invention will be made apparent by the following detailed description of several preferred embodiments of the inventive process. The description makes reference to the accompanying drawings in which:

FIGURE 1 is a schematic view of apparatus for practicing the first stage of my new process, that is, providing a hardened coating on the surface of the metallic P FIGURE 2 is a side view of apparatus for practicing the second stage of my new process, that is, burnishing and work hardening the coated metal surface;

FIGURE 3 is an end view of the apparatus of FIG- URE 2; and

FIGURE 4 is a view of alternate apparatus for practicing the second stage of my process.

As had been noted, the first stage of my process COIl-l sists of forming a hardened coating on a metallic surface by means of electrical discharges. This process has been disclosed in detail in the previously listed copending patent applications.

In essence this first stage utilizes an electrode 10 Which is connected to a power supply 12 so as to be positive with respect to the surface of a metallic object 14. In the particular embodiment illustrated, the electrode 10 is supported on an arm 16 which is in turn pivotably mounted about a support 18. A coil spring 20 which is connected between the support 18 and the rod 16 tends to force the electrode 10 into contact with the surface of the workpiece. A linear actuator 21 which might take the form of a pneumatic or hydraulic ram is connected to the arm 16. The linear actuator 21 is energized so as to cause it to reciprocate to slide the electrode 10 about the surface of the workpiece.

A vibratory mechanism 23 is fixed to the underside of the arm 16 at the end opposite to the electrode 10. It comprises an electric coil with iron core which may be caused to vibrate under suitable electric energization because of its attraction to metal of the arm 16. This vibration causes the arm 16 to vibrate about its support 18 and imparts a motion to the electrode 10 which is generally perpendicular to the workpiece surface.

The vibratory motion may be selectively employed with or without the sliding action caused by actuator 21 to treat a surface.

In general a smoother finish will be obtained with a pure sliding motion but the vibrating motion is necessary to reach some areas such as steep concavities as edges and when a high surface to electrode conductivity tends to short the power supply.

The workpiece surface 14 may be coated with a slurry containing carbon and fine carbide particles and/or the electrode may be formed of a carbon or carbide material which is capable of coating the surface. The pressure applied to the electrode 10 by the spring 20 should be sufficient to maintain the electrode in contact with the work surface, but not so high as to provide a direct short across the power supply. The electrode may be moved with respect to the workpiece at a speed in the range of approximately 1-10 inches per second. The power supply may provide approximately 10 to 250 volts at between .5 and 300 amperes.

As the workpiece 'is moved relative to the electrode, localized electrical discharges occur and fuse a coating material on the upper surface of the workpiece. As has been indicated, the heating effect softens the sub-surface zone and when steel is being treated produces a high austenitic content in this region. The surface coating produced is also relatively irregular because of the localized nature of the electrical discharges.

The second stage of my process which work hardens the austenitic sub-surface zone so as to actually convert the metal therein to martensite and which smooths the upper layer may be performed with a wide variety of apparatus and that illustrated in FIGURES 2 and 3 has been found to be effective. The coated surface of the workpiece is contacted with a roller 22 which is supported about a shaft 24 in a yoke type handle 26. The roller 22 is formed of a very hard metal or other material, preferably a carbide alloy. The roller is pressed against the workpiece and moved about the surface. Because only a line of contact exists between the workpiece and the surface, a relatively high pressure may be developed and I have found that a force of 100 pounds exerted on a roller 0.125 inch thick and 1 inch in diameter will produce an effective treatment of the previously hardened surface. It should be understood that the roller 22 might also be supported in some mechanized device which would move about the surface of the workpiece under uniform pressure, such as might be applied by spring pressure.

The roller might also be fixed in the handle 26 so that it is slid along the workpiece surface rather than rolled.

A surface lubricant such as molybdenum disulphide has been found to improve the efficiency of the operation.

FIGURE 4 illustrates an alternate method of treating the hardened surface. A workpiece 14 is subjected to treatment by a burnishing tool 28 which takes the form of an elongated cylinder of very hard metal or other material (preferably carbide) with a smooth rounded end. The tool 28 may be supported in the appropriate holder 30. This tool contacts the work with a very small area or a point contact, thereby developing relatively high pressures when only moderate forces are exerted on the handle either manually or by a machine. The tool 28 is again moved about the surface of the workpiece so as to contact all of the area thereof. It might also be rolled about the workpiece surface.

In one use of the tool such as shown in FIGURE 4 a one inch diameter tool had a rounded end such that its contacting region with the workpiece had a radius of approximately $4 of an inch. Pressure between 50 and 4 lbs. exerted on the tool was sufficient to obtain the desired results. Higher pressures will also work. The surface was fine and the tool :Was held at a 45 angle and subjected to reciprocating sliding motion.

In some areas such a tool cannot be moved along the workpiece surface with a sliding or rolling motion, as for example when sharply concave areas. In such cases the tool may be subjected to vibration at any desired frequency. For example from 20-200 cycles per second. The repeated impact of the tool flattened the high peaks on the surface. The finish obtained by a vibratin tool will be a function of its impacting force and the speed at which the tools are advanced on the surface.

It will be understood that while the invention has been described with particular reference to preferred embodiments thereof, various modifications may be made all within the scope of the claims which follow.

The embodiments of the invention in which exclusive property or privilege is claimed are defined as follows.

l. A process for forming a hard surface on a metal workpiece comprisingn the steps of contacting the workpiece with an electrode, there being a potential difference between said electrode and said workpiece with resultant intense localized electrical heating such that a hard relatively rough surface on said workpiece is provided; and then applying pressure to said hard surface, after it has cooled, by a dull hard tool which is moved over and in contact with the workpiece surface under pressure whereby at least portions of said hard rough surface are cold worked into the metal of said workpiece and said surface is made smoother.

2. A process as set forth in claim 1 wherein said hard surface results from deposition on the workpiece of a hard material during the treatment with said electrode.

3. A process as set forth in claim 1 wherein a lubricant is applied to said hard relatively rough surface prior to applying said pressure.

4. A method as set forth in claim 1 wherein the pressure is applied by causing said tool to rotate in contact with said surface.

5. A method as set forth in claim 4 wherein the pressure is applied by rolling said tool over said surface.

6. A method as set forth in claim 1 wherein the pressure is applied by sliding the tool over said surface.

7. The process of forming a hardened surface on a metal workpiece which comprises: bringing an electrode into position with respect to the workpiece surface; establishing a potential difference between the electrode and the workpiece and providing relative motion between the electrode and the workpiece so that the electrode is brought into contact with successive areas of the workpiece and localized electrical discharges occur adjacent the points of contact such that there is provided a hard relatively rough surface; and then [cold] working at least portions of said hard surface into said metal by applying pressure to the hard surface with a smooth, dull, hardened tool which is moved with respect to the Workpiece surface.

8. A process as set forth in claim 7 wherein said hard surface results from deposition on the workpiece of a hard material during the treatment with said electrode.

No references cited.

RICHARD M. WOOD, Primary Examiner.

J. G. SMITH, Examiner, 

1. A PROCESS FOR FORMING A HARD SURFACE ON A METAL WORKPIECE COMPRISING THE STEPS OF CONTACTING THE WORKPIECE WITH AN ELECTRODE, THERE BEING A POTENTIAL DIFFERENCE BETWEEN SAID ELECTRODE AND SAID WORKPIECE WITH RESULTANT INTENSE LOCALIZED ELECTRICAL HEATING SUCH THAT A HARD RELATIVELY ROUGH SURFACE ON SAID WORKPIECE IS PROVIDED; AND THEN APPLYING PRESSURE TO SAID HARD SURFACE, AFTER IT HAS COOLED, BY A DULL HARD TOOL WHICH IS MOVED OVER AND IN CONTACT WITH THE WORKPIECE SURFACE UNDER 